Overlay correction by reducing wafer slipping after alignment

ABSTRACT

A method and apparatus for correcting overlay errors in a lithography system. During lithographic exposure, features being exposed on the wafer need to overlay existing features on the wafer. Overlay is a critical performance parameter of lithography tools. The wafer is locally heated during exposure. Thermal expansion causes stress between the wafer and the wafer table, which will cause the wafer to slip if it exceeds the local frictional force. To increase the amount of expansion allowed before slipping occurs, the wafer chuck is uniformly expanded after the wafer has been loaded. This creates an initial stress between the wafer and the wafer table. As the wafer expands due to heating during exposure, the expansion first acts to relieve the initial stress before causing an opposite stress from thermal expansion. The wafer may be also be heated prior to attachment to the wafer chuck, creating the initial stress as the wafer cools.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to photolithography. Moreparticularly, the present invention relates to wafer alignment in aphotolithographic system.

2. Related Art

Photolithography (also called microlithography) is a semiconductordevice fabrication technology. Photolithography uses radiation, such asultraviolet or visible light, to generate fine patterns in asemiconductor device design. Many types of semiconductor devices, suchas diodes, transistors, and integrated circuits, can be fabricated usingphotolithographic techniques. Exposure systems or tools are used toimplement photolithographic techniques, such as etching, insemiconductor fabrication. An exposure system typically includes anillumination system, a reticle (also called a mask) or spatial lightmodulator (SLM) for creating a circuit pattern, a projection system, anda wafer alignment stage for aligning a photosensitive resist-coveredsemiconductor wafer. The illumination system illuminates a region of thereticle or SLM with a preferably rectangular slot illumination field.The projection system projects an image of the illuminated region of thereticle circuit pattern onto the wafer.

As semiconductor device manufacturing technology advances, there areever increasing demands on each component of the photolithography systemused to manufacture the semiconductor device. This includes increasingdemands on the accuracy of the wafer alignment. A wafer is typicallymounted on a wafer chuck, also referred to as a wafer table. Duringexposure, the features being exposed on the wafer need to overlayexisting features on the wafer. To achieve overlay performance, thewafer is aligned to the wafer stage prior to exposure. Any movement ofthe wafer relative to the wafer stage after alignment results in overlayerrors.

During exposure, the wafer is heated locally due to the energytransferred to the wafer from the exposure beam. This heating causes thewafer to expand. If the wafer expansion is unchecked, the expansionexceeds overlay error requirements. Clamping the wafer to the waferchuck reduces the amount the wafer expands. The wafer chuck is typicallydesigned to have a larger thermal mass than the wafer and ismanufactured of a material which has very low thermal expansion. Thisresults in relatively little expansion of the wafer chuck relative tothe wafer. The wafer chuck is also typically designed to be much stifferthan the wafer, such that if the wafer is sufficiently clamped to thewafer chuck, the thermal expansion of the wafer is reduced.

If the clamping force between the wafer and the wafer chuck is notsufficient to prevent wafer expansion, the wafer can slip on the waferchuck and larger wafer expansion will occur, resulting in larger overlayerrors. Slipping due to wafer expansion can be reduced by tightlyclamping the wafer to the surface of the wafer chuck with a vacuum. Thiscreates a frictional force between the wafer and the wafer chuck.However, if the wafer expansion force exceeds the frictional force, thewafer will slip, causing an overlay error. In extreme ultraviolet(“EUV”) systems, the chances of slipping increase because theenvironment surrounding the wafer during exposure is also a vacuum.Electrostatic clamping, which is much weaker than vacuum clamping, mustthus be used in lieu of a vacuum clamp.

Therefore, what is needed is a system and method for reducing theeffects of wafer expansion during exposure.

SUMMARY OF THE INVENTION

The present invention reduces wafer slipping by uniformly expanding thewafer chuck after the wafer has been attached. This creates an initialstress on the interface between the wafer and the wafer chuck, ratherthan a zero stress interface. Because the wafer chuck expands inrelation to the wafer, the initial stress is opposite that caused bywafer expansion during exposure. As the wafer heats up from exposure,the initial stress will first be reduced to a zero-stress interface.Only after this point will the expansion of the wafer create anexpansion stress on the interface between the wafer and wafer chuck.Ideally, the amount of heating without wafer slipping could be doubledwith the present invention.

The wafer table expansion can be achieved in several ways. In oneembodiment, a sealed circular tube, or annular ring, is attached to thecircumference of the wafer chuck. The annular ring is then pressurized.The annular ring expands when pressurized, thereby expanding the waferchuck to which it is attached. In a similar embodiment, the annular ringis not attached to the edge of the wafer chuck, but is embedded insidethe wafer chuck through a groove or cavity.

In another embodiment, a plurality of force actuators is attached to theedge of the wafer chuck. These force actuators act on the wafer chuck toexpand it.

The expansion of the wafer chuck can also be thermally induced. In oneembodiment, a heater is directly attached to the wafer chuck. In anotherembodiment, a proximity heater is placed near the wafer chuck. In stillanother embodiment, the wafer chuck is made out of an electricallyconductive material, and is connected to a power source. In a furtherembodiment, the wafer is heated before attachment, so that it is warmerrelative to the wafer chuck. In this embodiment, when the wafer isattached to the wafer chuck, they reach a thermal equilibrium. As thewafer cools, it contracts and thus creates an initial stress oppositethat of an expansion stress. The expansion from each of theseembodiments results in nearly uniform expansion of the wafer, similar toan overall magnification of the wafer, and thus can be compensated forin lithographic exposure tools.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 is a flowchart of a method according to an embodiment of thepresent invention.

FIG. 2A is an illustration of a wafer attached to a wafer chuck.

FIG. 2B is an illustration of a wafer chuck expanding in relation to awafer.

FIG. 2C is an illustration of a wafer attached to an expanded waferchuck.

FIG. 2D is an illustration of a wafer expanding in relation to anexpanded wafer chuck.

FIG. 3A is an illustration of an expansion system according to anembodiment of the present invention.

FIG. 3B is a cross-section illustration of the expansion system shown inFIG. 3A.

FIG. 4A is an illustration of another expansion system according to anembodiment of the present invention.

FIG. 4B is a cross-section illustration of the expansion system shown inFIG. 4A.

FIG. 5 is an illustration of another expansion system according to anembodiment of the present invention.

FIG. 6A is an illustration of an expansion system using a heateraccording to an embodiment of the present invention.

FIG. 6B is an illustration of another expansion system using a heateraccording to an embodiment of the present invention.

FIG. 6C is an illustration of another expansion system using a heateraccording to an embodiment of the present invention.

FIG. 6D is an illustration of another expansion system using a heateraccording to an embodiment of the present invention.

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

In current lithography systems, wafer slipping is reduced by tightlyclamping the wafer to the surface of a wafer chuck. One method ofclamping the wafer is by creating a vacuum between the wafer and thewafer chuck. This works because there is a pressure differential betweenthe vacuum and the surrounding environment. In extreme ultra-violet(“EUV”) lithography, however, the environment surrounding the waferduring exposure is also a vacuum. This prevents using a vacuum as aclamping force.

Alternatively, electrostatic clamping is used to clamp the wafer to thewafer chuck. A disadvantage of typical electrostatic clamping is thatthe amount of force achieved with electrostatic clamping is inherentlylimited. Electrostatic clamping is also related to the time taken toclamp and release the wafer. As a result, electrostatic clamping tendsto provide between 1/10 and 1/15 the clamping force of vacuum clamping.This means that the frictional force between the wafer and the waferchuck also decreases to 1/10 to 1/15 the frictional force in a vacuumsystem.

In most systems, the interface between the wafer and the wafer chuck isapproximately a zero stress interface at the time of clamping. Thismeans that there is no force on the interface to counteract thefrictional force between the wafer and the wafer chuck. When the waferis exposed, energy in the exposure beam heats the wafer and causes thewafer to expand. When some parts of the wafer are being exposed whileothers are not, the expansion causes the wafer to slip if it is notsufficiently clamped. Slipping occurs because the thermally-inducedexpansion stress exceeds the frictional force holding the wafer inplace. This introduces error into the system. In a system whereelectrostatic clamping is used, the frictional force is low, and theexpansion stress does not need to be very large to overcome thefrictional force.

For EUV systems, wafer heating is likely to be higher than in non-EUVsystems. This is a result of a significant amount of non-exposure energyincluded in the EUV exposure beam being transferred to the wafer. Mostof this energy is in the form of infrared (“IR”) radiation. In someexposures, IR energy at the wafer may equal that of the energy needed toexpose the wafer (also referred to as the “dose energy”). Thiseffectively doubles the heating at the wafer compared to non-EUVsystems.

These factors result in a situation where there is 1/10 to 1/15 theresistance to slipping as compared to a vacuum system, and twice theheating. Wafer slipping thus becomes much more likely, causing the doselimit determined by overlay to be small in comparison with the doseallowed for non-EUV systems.

FIG. 1 is a flowchart of a method 100 according to an embodiment of thepresent invention. Method 100 allows application of a larger dose energybefore wafer slipping becomes a threat. Although the present inventionwill be described herein with reference to EUV systems usingelectrostatic clamping, one skilled in the art will recognize that thepresent invention may also be used in non-EUV systems and/or lithographysystems using clamping methods other than electrostatic clamping.

In step 102, a wafer is attached to a wafer chuck in a lithographysystem. In one embodiment, the wafer is attached using electrostaticclamping. In another embodiment, the wafer is attached using vacuumclamping.

In step 104, the wafer chuck is uniformly expanded. This creates aninitial stress on the interface between the wafer and the wafer chuck.During exposure, due to heat transfer, the size of a wafer increaseswith respect to the wafer chuck. By expanding the wafer chuck prior toexposure, the size of the wafer is effectively decreased with respect tothe chuck. Therefore, the initial stress caused by wafer chuck expansionis opposite the stress caused by wafer expansion. The initial stress maybe almost equal to the frictional force between the wafer and the waferchuck. In this embodiment, additional stress would overcome thefrictional force, and cause the wafer to slip prior to exposure.However, by keeping the initial stress just below the magnitude of thefrictional force, slippage is prevented.

In step 106, the wafer is aligned to the wafer stage of a lithographysystem. This alignment centers the wafer in an exposure beam path, andensures proper focus and alignment of a lithography pattern duringexposure.

In step 108, the wafer is exposed, causing the wafer to expand. Becausethe expansion stress is opposite that of the initial stress, expandingthe wafer first acts to relieve the initial stress. Only after relievingthe initial stress is a new expansion stress created as a result of thewafer getting larger with respect to the wafer chuck.

If the initial stress is almost equal to the frictional force betweenthe wafer and the wafer chuck, the exposure dose may be almost doubledcompared to a system having no initial stress.

FIGS. 2A to 2D illustrate the succession of method 100. FIG. 2Aillustrates a wafer 202 attached to a wafer chuck 204 (not to scale).Wafer 202 is attached via a clamping method, such as vacuum clamping or,in a preferred embodiment, electrostatic clamping.

FIG. 2B illustrates the expansion of wafer chuck 204, as occurs in step104. Wafer chuck 204 expands uniformly in all directions, as indicatedby arrows 206. During expansion, the size of wafer chuck 204 increaseswith respect to wafer 202.

FIG. 2C illustrates a configuration of wafer 202 and wafer chuck 204immediately before wafer 202 is exposed. Because of the wafer chuckexpansion, there is an initial stress between wafer 202 and wafer chuck204. Because wafer chuck 204 attempts to stretch wafer 202 as waferchuck 204 expands, the initial stress on wafer 202 may be referred to asan outward force.

FIG. 2D illustrates the expansion of wafer 202 due to heating duringexposure. As wafer 202 increases in size, the outward force is relieved.At some point during the wafer expansion, wafer 202 reaches a pointwhere there is zero stress between wafer 202 and wafer chuck 204. Ifwafer 202 continues expanding past this point, an inward force iscreated on wafer 202. As long as the magnitude of this inward force doesnot exceed that of the frictional force between wafer 202 and waferchuck 204, wafer 202 will not slip. Wafer chuck 204 also expands due toheating during exposure. The expansion of wafer chuck 204 lessens theexpansion rate of wafer 202 relative to wafer chuck 204. Thus, themagnitude increase of the inward force created on wafer 202 is alsolessened.

In a preferred embodiment, wafer 202 is a round wafer. In thisembodiment, wafer 202 expands uniformly while heating. Since theexpansion is uniform, there is no imbalance of the inward force on wafer202, and the likelihood of slipping is lessened. Further, if the waferexpands uniformly, the exposure pattern can be magnified to compensatefor the change in size. Compensation would be difficult if sections ofthe wafer expanded non-uniformly.

FIG. 3A is an illustration of an embodiment of a system of the presentinvention. An annular tube 302 is attached to the outside of wafer chuck204. In one embodiment, annular tube 302 is a metal tube. In anotherembodiment, annular tube 302 is manufactured from a plastic. Annulartube 302 includes a cavity 306. Cavity 306 can be filled with eitherliquid or gas. When annular tube 302 is pressurized, it expands. Sinceannular tube 302 is attached to the edge of wafer chuck 204, wafer chuck204 uniformly expands with it. FIG. 3B is a cross-section of theillustration in FIG. 3A, taken at line 304. As shown, annular tube 302is attached to the edge of wafer chuck 204.

FIG. 4A is an illustration of another embodiment of the presentinvention. Similar to the above embodiment, an annular ring 402 havingcavity 406 is attached to wafer chuck 204. In this embodiment, annularring 402 is attached inside a cavity or groove in wafer chuck 204.Because annular ring 402 is embedded into wafer chuck 204, there is alesser chance of annular ring 402 detaching from wafer chuck 204. Inaddition, non-uniformities caused by materials used to attach theannular ring to the edge of wafer chuck 204 are avoided. FIG. 4B is across-section of the illustration in FIG. 4A, taken at line 404. In oneembodiment, annular ring 402 is fully enclosed in the structure of wafer204, as shown in FIG. 4B. In another embodiment, annular ring 402 isonly partially embedded in wafer chuck 204, as in a groove.

FIG. 5 is an illustration of another embodiment of the presentinvention. In this embodiment, a plurality of force actuators 502 areattached on one end to a fixed support 504, and on the other end towafer chuck 204. The actual number of force actuators 502 used isvariable. In one embodiment, force actuators 502 are distributed evenlyand symmetrically around wafer chuck 204. When force actuators 502 areactivated, they pull on wafer chuck 204. In this manner, force actuators502 act together to exert a uniform force on wafer chuck 204 that isoutward with respect to wafer chuck 204. This outward force causesuniform expansion of wafer chuck 204.

FIGS. 6A through 6D illustrate various embodiments of the presentinvention in which wafer chuck 204 is expanded through heating. In theembodiment of FIG. 6A, a contact heater 602 directly heats wafer chuck204. As the temperature of wafer chuck 204 increases, wafer chuck 204expands. Contact heater 602 may be as large as needed in comparison towafer chuck 204 to cause uniform heating and expansion throughout waferchuck 204. Although this embodiment provides direct heat flow, theattachment of heater 602 may inhibit the expansion of wafer chuck 204.

FIG. 6B illustrates another embodiment of the present invention. In thisembodiment, a proximity heater 604 is placed near a surface of waferchuck 204. Proximity heater 604 may heat via thermal or electromagneticradiation. Since proximity heater 604 does not come in contact withwafer chuck 204, it does not inhibit expansion of wafer chuck 204. Asshown in FIG. 6C, proximity heater 604 may vary in size and heatdistribution as needed for uniform expansion of wafer chuck 204.

FIG. 6D illustrates another embodiment of the present invention in whicha heater is used. In this embodiment, wafer chuck 204 is manufacturedfrom an electrically conductive material. A power source 606 isconnected to wafer chuck 204 by leads 608 and 610. As power passesthrough wafer chuck 204, it heats up and expands. Leads 608 and 610 maybe flexible so as to allow the free expansion of wafer chuck 204. Powersource 606 may be a variable power source.

In a further embodiment, wafer 202 is heated before being attached towafer chuck 204, so that it is warmer than wafer chuck 204. In thisembodiment, when wafer 202 is attached to wafer chuck 204, they reachthermal equilibrium. As wafer 202 cools, it contracts and thus createsan initial stress opposite that of an expansion force.

In each of the heating embodiments, a temperature sensor can be mountedon wafer stage 204 to monitor the expansion. A control circuit may beattached to the heater to precisely control or adjust the heatingprocess.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A method of reducing wafer slipping in a lithography system having awafer chuck, comprising: a. attaching a wafer to the wafer chuck tocreate an interface between the wafer and the wafer chuck; b. expandingthe wafer chuck to create an initial stress on the interface; c.aligning the wafer and chuck to a wafer stage; and d. exposing the waferto a radiation source.
 2. The method of claim 1, wherein said initialstress is sufficient to prevent slipping of the wafer during saidexposing step.
 3. The method of claim 2, wherein said expanding stepcomprises pressurizing an annular ring attached to the wafer chuck. 4.The method of claim 2, wherein said expanding step comprises actuating aforce on the wafer chuck in a direction outward from the wafer chuck. 5.The method of claim 2, wherein said expanding step comprises heating thewafer chuck.
 6. The method of claim 2, wherein said expanding stepexpands the wafer chuck uniformly in all directions.
 7. A method ofperforming lithography in a vacuum, said method comprising: a. attachinga wafer to a wafer chuck, to create an interface between the wafer andwafer chuck; b. aligning the wafer to the wafer chuck; c. expanding saidwafer chuck to create an initial stress on the interface between thewafer and the wafer chuck; and d. exposing said wafer to a radiationsource, wherein said initial stress on the interface between said waferand said wafer chuck prevents slipping of said wafer during saidexposing step.
 8. The method of claim 7, wherein said expanding stepcomprises heating said wafer chuck.
 9. The method of claim 7, whereinsaid expanding step comprises exerting a force on the edges of saidwafer chuck, wherein said force is outward relative to said wafer chuck.10. The method of claim 7, wherein said exposing step causes said waferto expand.
 11. A method of reducing overlay in a lithography systemhaving a wafer chuck, comprising: a. heating a wafer, to cause the waferto expand in size; b. attaching the wafer to the wafer chuck; c.allowing the wafer to cool, thereby contracting in size and creating aninitial stress between the wafer and the wafer chuck.
 12. A lithographysystem configured to reduce wafer slipping, comprising: a. a wafer chuckconfigured to receive a wafer; and b. an expander coupled to the waferchuck to expand the wafer chuck and create an initial stress at aninterface between the wafer and the wafer chuck.
 13. The system of claim12, wherein said expander comprises: an annular ring coupled to saidwafer chuck.
 14. The system of claim 13, wherein said annular ring iscoupled to an outer edge of said wafer chuck.
 15. The system of claim13, wherein said annular ring fits within a cavity in said wafer chuck.16. The system of claim 12, wherein said expander comprises: a pluralityof force actuators attached to said wafer chuck, wherein said pluralityof force actuators is configured to stretch said wafer chuck.
 17. Thesystem of claim 16, wherein said plurality of force actuators isdispersed evenly around said wafer chuck.
 18. The system of claim 12,wherein said expander comprises: a heater configured to heat said waferchuck.
 19. The system of claim 18, wherein said heater is a contactheater disposed in contact with said wafer chuck.
 20. The system ofclaim 18, wherein said heater is a proximity heater spaced apart fromsaid wafer chuck.
 21. The system of claim 18, wherein said heatercomprises an electrical current source coupled to said wafer chuck topass an electric current through said wafer chuck.
 22. The system ofclaim 18, wherein said heater heats said wafer chuck withelectromagnetic radiation.
 23. The system of claim 12, wherein saidexpander is configured to expand said wafer chuck in a uniform manner.